[BUGFIX] Undefined array key "contentSecurityPolicies"

If the 'contentSecurityPolicies' value is not filled properly
in the site configuration, the call to
$site->getConfiguration()['contentSecurityPolicies'] will fail.
The access should we guarded with a null coalescing operator.

Resolves: #104873
Releases: main, 12.4
Change-Id: Ie49a25d8ca9a194629c4a389b17b299091d44031
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85977


Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Benni Mack <benni@typo3.org>